This paper presents an explanatory overview of a large and complex grammar, DIAGRAM, that is used in a computer system for interpreting English dialogue. DIAGRAM analyzes all of the basic kinds of phrases and sentences and many quite complex ones as well. It is not tied to a particular domain of application, and it can be extended to analyze additional constructions, using the formalism in which it is currently written. For every expression it analyzes, DIAGRAM provides an annotated description of the structural relations holding among its constituents. The annotations provide important information for other parts of the system that interpret the expression in the context of a dialogue. DIAGRAM is an augmented phrase structure grammar. Its rule procedures allow phrases to inherit attributes from their constituents and to acquire attributes from the larger phrases in which they themselves are constituents. Consequently, when these attributes are used to set context-sensitive constraints on the acceptance of an analysis, the contextual constraints can be imposed by conditions on dominance as well as conditions on constituency. Rule procedures can also assign scores to an analysis, rating some applications of a rule as probable or as unlikely. Less likely analyses can be ignored by the procedures that interpret the utterance.